The effects of urban noise, dispersal and isolation on vocalizations of the silvereye (Zosterops lateralis)

Abstract

Birdsong as an acoustic signal is subject to many different environmental pressures, including noise. The ability of songbirds to shift songs to transmit effectively in these environments may be flexible, however innate calls may be more difficult to change and these differences can reflect the different roles that environment, morphology and genetics might play in determining the structure of vocalizations. In this study, I have compared songs and calls from populations of silvereyes (Zosterops lateralis) inhabiting different environments across Australia and on islands in the Pacific. Results show taht in an urban context, environmental factors such as noise play an important role in shaping both structural and spectral characteristics of songs and calls in this species. In cities, silvereyes sing and contact call at higher frequencies than in rural areas, and these differences are also associated with differences in the use of repertoire elements. Urban environments may therefore be selecting for certain song dialects. Further evidence shows that frequency shifts may be attributable to flexible, real-time responses to the current noise environment: in a captive experiment, both urban and rural birds shifted calls immediately in response to noise at various frequencies, likely to increase transmission. Despite this, urban vocalizations may not all be affected in the same way - alarm calls tend to be lower in urban areas, which suggests that acoustic adaptation predictions may be highly dependent on context.

While differences in songs and calls are not associated with neutral genetic or basic morphological change in cities, morphology and dispersal may play a part in determining the structure of repertoires, songs and calls in silvereyes inhabiting isolated islands across the Pacific. Data comparing multiple isolated populations reveal that the larger body size of silvereyes on islands may be associated with a lowering of vocalization frequency, however specific dialects or repertoires are predicted by spatial connectivity, genetic connectivity and habitat factors. Together, this string of evidence provides support for multiple, context-dependent theories behind the evolution of bird song characteristics, including cultural drift and most prominantly, the acoustic adaptation hypothesis.